Pharmaceuticals and other materials are often dried in fluid bed dryers and it is known to monitor the progress of the drying using an infrared spectrometer (commonly referred to as a “gauge”). The gauge projects a beam of infrared radiation through a window in the wall of the dryer and measures the light reflected by the material being dried. The radiation directed into the dryer includes (a) a wavelength that is absorbed by water and (b) a wavelength that is not absorbed by water or the material. The intensity of radiation reflected back by the material being dried in the wavelength that is absorbed by water will be reduced as a result of absorption by water in the material in comparison to the intensity of the reflected radiation in wavelengths that are not absorbed. The intensity of light at that wavelength increases as the amount of water in the material decrease. By comparing the intensity of the radiation at the absorbed wavelength with that at non-absorbed wavelengths, it is possible to measure the amount of water in the pharmaceutical and assess when it is dry. The window should have a high transmission for radiation in all the wavelengths being used for measurement.
Suitable infrared gauges for such an application are well known and are manufactured by the Applicant Company. An example of such a gauge is described in published PCT application no. W098/22806. However, the precise details of the gauge do not form part of the present invention.
WO01/25153 describes a radiation sensor mounted in a cylindrical body for sensing radiation from a source. The sensor is mounted in a channel with the axis of the cylindrical body lying at right angles to the axis of the channel. The cylindrical body can be rotated through 180° to a position facing away from the radiation source so that the sensor can be cleaned.
JP-11-288870 describes a system for cleaning lenses. Two sets of lenses are provided on a rotatable housing. When one set of lenses is in use, the other is located at a cleaning station. The housing can be rotated to bring the lenses that were previously in use into the cleaning station and vice versa for the other set of lenses. Such an arrangement doubles the cost of the lenses.
GB-1567031 describes a sensor for sensing the turbidity of a liquid and includes a light source and a light sensor. The light source and light sensor can be moved into a retracted position past wiper blades to clean any deposits on the light transmitting housing containing the sensor and/or light source.
GB-2332756 describes a camera attachment for location in front of the camera lens. It consists of a disc that is rotated at high speed to remove any moisture that may gather on it.
WO89/09388 describes a near infrared analysis device of material held in a test chamber, which has an opening through which infrared radiation can pass. A ribbon of infrared transparent material is pressed against the opening to form a window. Between successive samples, the ribbon is advanced so that a fresh piece of ribbon forms the window in each successive test.
U.S. Pat. No. 5,161,055 describes a rotating inspection window that spins at such a speed that water or other debris falling on the window is slung off by centrifugal force.
U.S. Pat. No. 4,260,262 describes a grain quality analyser in which the grain is successively exposed to radiation of different wavelengths, including infrared.
CH-591078 describes a device for measuring the purity of a liquid flowing through a pipe. Part of the pipe is replaced by a light-transmitting sleeve that enables the liquid flowing through the sleeve to be analysed photometrically. Wiper blades are provided on the inside of the sleeve and the sleeve is rotated so that the blades wipe the inside of the sleeve clean.
JP-11118714 describes a system for measuring turbidity having a light source and a light sensor. A disc is rotated between the source and the sensor.
JP-1100272 descibes a vapour deposition apparatus having an energy source that decomposes reaction gas. The radiation source is located within a cylindrical transparent window that is rotated so that part of its circumference is located in a vapour deposition chamber and part in a cleaning chamber.
JP-827824 describes a powder treatment apparatus having a light transmitting window in the form of a rotatable cylinder that can be rotated past a wiper to clean the drum surface.
One severe disadvantage that limits the use of infrared gauges in the monitoring of materials being dried in fluid bed and other dryers is that the material can be sticky, particularly when moist. It can therefore adhere to the window and prevent radiation from passing into the interior of the dryer. Since, in these circumstances, it is not possible to monitor the water content of the material being dried, the drying may have to be stopped in order to allow the window to be cleaned.
The present invention is intended to overcome the above problem.